Solar panel circuit MCB size?

[driverman]

Hi guys,
I have asked this in another Electricians thread here. Firstly what size MCB usually required for lets say a 4kw solar panel load? Also it has been mentioned about RCD's protecting the solar panel circuit may not be a good idea. Any thoughts please? It's a brand new house build.i was planning to install RCBO's to each and every circuit. Thanks http://www.electriciansforums.co.uk...orum/98430-solar-panels-circuit-mcb-size.html

 

[Leesparkykent]

#14 in your last thread and a 16amp normally for a 4kw system. What more clarity do you want? Lol

 

[driverman]

#14 in your last thread and a 16amp normally for a 4kw system. What more clarity do you want? Lol

Just bored at work! It has come to light also about RCD protection. so thought I'd try this forum who put Solar panels in daily. But i have taken on board what has been said. thanks

 

[whinmoor]

See below for info from Samil's product manual. Other inverter manufacturers may have different ideas.

 

[BruceB]

A 16 A mcb is a reasonable starting point for a system that has a nominal 16 A output.
But it could mean running the mcb near maximum for many hours in the summer.
When in doubt read the manual!
ABB recommend 20 A for the 3.6TL and 25 A for the 4.2TL
SMA recommend up to 32 A for their range from 3000TL to 5000TL

 

[davesparks]

But it could mean running the mcb near maximum for many hours in the summer.

Is there a problem with that?

 

[whinmoor]

Is there a problem with that?

The could be a problem with discrimination...

 

[Leesparkykent]

Discrimination of what?

 

[BruceB]

There can be a problem with mcbs running hot near maximum rated current. Manufacturers' websites go into more detail. As the cable size for 4 kW PV is generally recommended to be 4 mm2 or larger, there is normally scope for a larger mcb.

 

[driverman]

I will be installing 6mm as I have that on site. No point getting 4mm. Any thoughts about 30mA protection? Am I asking for trouble?

I could always install a 20A MCB if 16A near the limit.

 

[davesparks]

The could be a problem with discrimination...

How does discrimination change due to the amount of current flowing through the mcb?

 

[driverman]

See below for info from Samil's product manual. Other inverter manufacturers may have different ideas.


View attachment 27461

Thanks, according to graph, i could in fact use 2.5mm cable as the inverter is next to the consumer unit. Having said that, installers mentioned 4mm. I've got 6mm, so will use that. I doubt if the circuit will more than 1m.

 

[Leesparkykent]

What about the dc side of the installation? How's this being run?

 

[whinmoor]

"Thanks, according to graph, i could in fact use 2.5mm cable as the inverter is next to the consumer unit. Having said that, installers mentioned 4mm. I've got 6mm, so will use that. I doubt if the circuit will more than 1m."

No, the graph is for the 3300TL model and is given as an example only!! 6mm cable is definitely fine though in a 4kWp domestic situation.

 

[driverman]

What about the dc side of the installation? How's this being run?

The DC side of the installation will be run in galvanised 20mm steel conduit which has been flushed into the corner (150mm). Inverter will be housed on ground floor. 2 x isolators have been installed in the loft by the installers.

 

[Gavin A]

A 16 A mcb is a reasonable starting point for a system that has a nominal 16 A output.
But it could mean running the mcb near maximum for many hours in the summer.
When in doubt read the manual!
ABB recommend 20 A for the 3.6TL and 25 A for the 4.2TL
SMA recommend up to 32 A for their range from 3000TL to 5000TL

I don't think we've had a single case of nuisance tripping on any of the 16amp MCB's we've installed on 16A generation limited systems, which would be all of those hundreds of systems.

I've had to swap out a couple of 30mA RCDs for nuisance tripping, but that's it.

 

[davesparks]

Of course you won't have, a 16A mcb will handle 16A, that's the whole point of it being a 16A rated device.

 

[Leesparkykent]

The DC side of the installation will be run in galvanised 20mm steel conduit which has been flushed into the corner (150mm). Inverter will be housed on ground floor. 2 x isolators have been installed in the loft by the installers.

i think burying DC cables in the wall is an awful idea IMO

 

[driverman]

i think burying DC cables in the wall is an awful idea IMO

How would you normally install DC cables in a new build house? Surly, not surface in say trunking? The Solar people wanted it done that way and agreed the route which is basically straight up from main / inverter area to loft space. They wanted it in steel conduit. I went galvanised, even though it was behind dot dab. I even used female /male brass bushes. Would be interesting to know how you and others would do the job though?

 

[Leesparkykent]

i would of tried getting the inverter closer to the array or used micro inverters.

 

[driverman]

i would of tried getting the inverter closer to the array or used micro inverters.

I know nothing about solar installations, just doing to their spec. The distance between the two is not that great really. I would say the depth of a bedroom 3.4m ish plus another 3m. Would you consider that a long DC run? For whatever reason customer or installers wanted the inverter on the ground floor. Possibly for maintenance? Save going up into the loft? I don't know really,I'm just trying to get it done! lol

 

[Gavin A]

i think burying DC cables in the wall is an awful idea IMO

If it's mechanically protected what's the issue?

 

[shanky887614]

nothing wrong with it but i prefer the inverter up in the loft.

that way only the ac cable goes to meter then board

 

[TJ Anderson]

Of course you won't have, a 16A mcb will handle 16A, that's the whole point of it being a 16A rated device.

All well and good as long as G/83 settings applied which limit output. These inverters are made to work in different markets. We had an SMA TL 4.0 that went quite a bit over before it was realised.

 

[Leesparkykent]

Because in my view burying them in the wall even in galv conduit, customer turns main switch off in house for whatever reason they still have live cables running down the wall buried! the customer doesn't know they are there, only the person who done the install will.

 

[Gavin A]

nothing wrong with it but i prefer the inverter up in the loft. Reply With Quote

that way only the ac cable goes to meter then board

reasons not to would include the inverter operating at a higher ambient air temperature when at peak output, and therefore reducing the lifespan of the inverter (which ultimately is dictated by the lifespan of the capacitors), dustier environment, and the inverter being less accessible to check for repairs.

We'll install in lofts when there's no other option, but our preference is for the inverter in a garage or other accessible cooler space within the house if there is the option for those reasons.

 

[davesparks]

Because in my view burying them in the wall even in galv conduit, customer turns main switch off in house for whatever reason they still have live cables running down the wall buried! the customer doesn't know they are there, only the person who done the install will.

Then perhaps the customer should read the label at the CU which warns of the dual supply and the location of isolators required to achieve complete isolation

 

[Leesparkykent]

reasons not to would include the inverter operating at a higher ambient air temperature when at peak output, and therefore reducing the lifespan of the inverter (which ultimately is dictated by the lifespan of the capacitors), dustier environment, and the inverter being less accessible to check for repairs.

We'll install in lofts when there's no other option, but our preference is for the inverter in a garage or other accessible cooler space within the house if there is the option for those reasons.

agreed but could possibly go in a cuopboard upstairs etc, or just use enphase.

 

[Leesparkykent]

Then perhaps the customer should read the label at the CU which warns of the dual supply and the location of isolators required to achieve complete isolation

hows that going to stop the dc cables from being live? is there an isolator to turn the sun off? lol

 

[davesparks]

All well and good as long as G/83 settings applied which limit output. These inverters are made to work in different markets. We had an SMA TL 4.0 that went quite a bit over before it was realised.

I don't understand, how will the settings cause a 16A mcb to not be able to handle 16A? That is what my post stated, a 16A mcb will be fine with 16A flowing through it.

If what you are saying is that if the inverter could deliver more than 16A if not correctly set up then obviously that is a different scenario.

 

[davesparks]

hows that going to stop the dc cables from being live? is there an isolator to turn the sun off? lol

I don't know, I don't install solar. But I do have some common sense and that says you would have a DC isolator where the cables enter the property to isolate them.

 

[TJ Anderson]

hows that going to stop the dc cables from being live? is there an isolator to turn the sun off? lol

I was thinking the same, unless he has DC isolators upstream of the buried section. Unlikely though, probably just next to inverter.

 

[Gavin A]

Because in my view burying them in the wall even in galv conduit, customer turns main switch off in house for whatever reason they still have live cables running down the wall buried! the customer doesn't know they are there, only the person who done the install will.

again, if it's mechanically protected what's the problem?

and I mean mechanically protected with proper 3mm thick galv steel.

The stickers on the consumer unit, and fact there's an inverter downstairs and panels on the roof ought to give an indication that there are live dc cables in the wall. We'll always put a DC isolator in the loft as well so it can be isolated if needed.

 

[Gavin A]

hows that going to stop the dc cables from being live? is there an isolator to turn the sun off? lol

isolator in the loft.

 

[Leesparkykent]

I don't know, I don't install solar. But I do have some common sense and that says you would have a DC isolator where the cables enter the property to isolate them.

yea 9X out of 10 the dc cables will enter the loft so prob would be best to stick a dc isolator there also but the customer aint climbing in to the loft to isolate this are they! they will just crank the ac and dc isolator off at the inverter ( in this case its going to be next to the consumer unit) hence leaving the buried DC cables live in the wall when the main switch at consumer unit,ac isolator/dc isolator at the inverter are all off.

 

[TJ Anderson]

isolator in the loft.

Even if there was, which is unlikely if the inverter in elsewhere (very commendable of you though), the customer would just switch of Main AC Isolator for PV (by CU) and assume they had done it. Then they would still get a free crash course in arc welding when they get their drill out.

 

[davesparks]

yea 9X out of 10 the dc cables will enter the loft so prob would be best to stick a dc isolator there also but the customer aint climbing in to the loft to isolate this are they! they will just crank the ac and dc isolator off at the inverter ( in this case its going to be next to the consumer unit) hence leaving the buried DC cables live in the wall when the main switch at consumer unit,ac isolator/dc isolator at the inverter are all off.

Then if they can't read a label which clearly states how to isolate correctly what hope has the human race really got?
So what exactly is this scenario you are describing here, why is the customer trying to isolate the installation in the first place?

 

[Leesparkykent]

again, if it's mechanically protected what's the problem?

and I mean mechanically protected with proper 3mm thick galv steel.

The stickers on the consumer unit, and fact there's an inverter downstairs and panels on the roof ought to give an indication that there are live dc cables in the wall. We'll always put a DC isolator in the loft as well so it can be isolated if needed.

of course because it will always be an electrician who buys this new build house and knows the ins and outs of an electrical installation/PV system so will automatically assume all this. behave! lol

 

[Gavin A]

I don't understand what it is you think a house owner is going to be doing to result in them attacking the wall with such force that they can end up breaching the protection of heavy duty galv conduit before realising that they've hit something hard and metallic and should probably stop attacking it and work out what it is.

 

[Leesparkykent]

because diy dave has got his big shiny drill out on a sunday!

 

[Leesparkykent]

i give up to be honest, you have your opinions i have mine.

 

[Gavin A]

because diy dave has got his big shiny drill out on a sunday!

with the power off, and he just happens to be using a metal drill bit rather than a masonry or wood drill bit?

and is going to just keep drilling despite the drill not going in like it should and sounding like it's hit something metal?

 

[davesparks]

because diy dave has got his big shiny drill out on a sunday!

Which is unlikely to operate once he's turned all of the power off. Or if he's using a cordless then I doubt a DIY level cordless drill will be man enough to injure galv pipe.
But either way how many diyers actually switch off their whole house before drilling holes? All that will happen if they do is that it won't go bang when they drill through a cable but probably will when they turn it back on.

 

[davesparks]

with the power off, and he just happens to be using a metal drill bit rather than a masonry or wood drill bit?

and is going to just keep drilling despite the drill not going in like it should and sounding like it's hit something metal?

I have to say there are a fair few who do just keep going after they've hit a buried bit of metal, carpenters are quite good at that trick too, along with plumbers and kitchen fitters

 

[Leesparkykent]

with the power off, and he just happens to be using a metal drill bit rather than a masonry or wood drill bit?

and is going to just keep drilling despite the drill not going in like it should and sounding like it's hit something metal?

he is a DIY'er after all!

 

[davesparks]

Thinking about it I've been guilty of the 'get the SDS out and give it some proper welly' approach when encountering resistance whilst drilling a hole. I wouldn't be surprised if most of us are!

 

[Gavin A]

besides, with the inverter off, even transformerless inverter circuits become protected by dint of being electrically separated.

Switch it back on and the insulation resistance test on start up should detect a damaged cable and not start up, so the circuit stays electrically separated until the fault's repaired.

At the end of the day, it's allowed under BS7671, and it's allowed because it's considered to be safe.

 

[Leesparkykent]

Thinking about it I've been guilty of the 'get the SDS out and give it some proper welly' approach when encountering resistance whilst drilling a hole. I wouldn't be surprised if most of us are!

exactly this is my point, it has been known lol

 

[Leesparkykent]

besides, with the inverter off, even transformerless inverter circuits become protected by dint of being electrically separated.

Switch it back on and the insulation resistance test on start up should detect a damaged cable and not start up, so the circuit stays electrically separated until the fault's repaired.

At the end of the day, it's allowed under BS7671, and it's allowed because it's considered to be safe.

and that is why i said IMO

 

[shanky887614]

why would the customer need to go into the loft to isolate it anyway.

if the inverter is in the loft just isolating the ac side will turn the inverter off.

the dc cables will always have potential on them unless the customer covers them on the roof or works at night lol